February 2020 – TD 266 OPERATING MANUAL – ZPulse® DCS 4420/4830/4520/4930 Page 109
8.6 Checking the acoustics
The best way to check the acoustics is to do a short pre-deployment where the sensor is hanging from a floating raft or
boat. Checking the signal strength from the different beams gives an indication if some of the beams are much weaker
than the others. When checking the beam strengths close to the sensor, the signal strength should normally be within 2-
3dB between the different beams.
A weaker signal strength on all transducers than seen earlier is not necessarily an indication that something is wrong
with the sensor. The signal strength can show big variations during a year due to changes in biological activities, etc.
Some places variations of the backscatter conditions can give several dB in signal strength variation.
8.6.1 Test using Test Unit 5048
The Test Unit 5048 is designed to verify that vital parts of the ZPulse Doppler Current Sensor (DCS) are working correctly.
The Test Unit is designed for DCS installed on the SeaGuard SeaGuardII, RCM Blue and for stand-alone DCS in the 4420,
4520, 4830, 4930 series. For older DCS with single frequency use Test Unit 3731. The Test Unit consists of a ring with 4
test transducers suspended by mechanical springs, enabling each test transducer to be pressed against the DCS
transducers. The test transducers pick up some of the energy transmitted by the ‘ping’ from the DCS. This energy is used
to start oscillation of the test transducers. In the receiving stage of the DCS the test transducers are still oscillating and thus
transmitting a weak signal back to the DCS. Each of the four test transducers consist of 2 oscillators corresponding to the
two frequencies in each ZPulse signal. Two of the test transducers are made of crystals with slightly higher resonance
frequency than the DCS transducers, and two are made of crystals with slightly lower resonance frequency. The high
frequency corresponds to the received signal when the current direction is towards the DCS, and the low frequency
corresponds to a current flowing away from the DSC. The frequency shift corresponds to a simulated current speed of
about 2.20 m/s if Forward Ping is disabled. The direction of the simulated current is along the center line between the two
low frequency test transducers, marked by a slot in the Test Unit ring.
8.6.2 Procedure for mounting the Test Unit to the DCS:
•
Moisten the surface of the transducers for optimal contact.
•
Bring the test transducers to their outer position by pulling and turning the grey handles.
•
Hold the Test Unit around the DCS and align the test transducers to face the DCS transducers.
•
Release carefully the mechanical springs one by one so that the test transducers are in contact with the DCS
transducers.
•
Tilting the instrument/DCS corresponds to a slight increase in current speed readings:
•
10° tilt corresponds to 2% increase in reading.
•
35° tilt corresponds to 8% increase in reading.
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